KR100492112B1 - METHOD FOR CONTROLLING THE PRODUCTION OF AMMONIA FROM UREA FOR NOx SCRUBBING - Google Patents

Abstract

A process to provide a pressurized gas stream useful for removing nitrogen oxides from a combustion gas stream by hydrolyzing urea in aqueous solution in a closed reactor to evolve gaseous ammonia at a rate essentially balanced to the amount required from the combustion gas stream. The improvement resides in maintaining the pressure in the reactor within a preselected range when the demand for ammonia for external use suddenly drops by cooling the solution within the hydrolysis reactor by heat exchange either within or external to the reactor in response to rapid changes in demand for ammonia required to remove said nitrogen oxides.

Description

METHOD FOR CONTROLLING THE PRODUCTION OF AMMONIA FROM UREA FOR NOx SCRUBBING}

According to US Pat. No. 6,077,491, the disclosure of which is incorporated herein by reference in its entirety, nitrogen oxides in the combustion gas stream by a selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) nitrogen oxide (NO _x ) control method. The aqueous urea solution is converted into a gaseous product stream of ammonia and carbon dioxide at a rate essentially comparable to the amount of ammonia required for removal. This method is generally in accordance with the following general process conditions.

An aqueous solution of urea or a mixture of urea and urea precursors having a solid concentration of about 1% to about 76% by weight is fed to the reactor. Urea is hydrolyzed at a temperature of about 110 ° C. to about 300 ° C. and a pressure of about 20 to about 500 psig to produce a gaseous product stream of ammonia, carbon dioxide and water, and the temperature or pressure of the reaction mixture is controlled by the heat input. The heat input should be maintained at a level sufficient to meet the ammonia demand to the extent required to essentially reduce all the nitrogen oxides present in the combustion gas stream.

It is essential to carefully balance the amount of ammonia injected into the combustion gas stream with the amount required to remove nitrogen oxides. When excess ammonia is injected, it can be discharged into the flue gas stack to form pollutants. Problems with ammonia leaks are recognized, for example, in US Pat. No. 4,719,092 to Bowers, US Pat. No. 5,098,680 to Fellows and US Pat. No. 5,237,939 to Spokoyny. come. However, all of these patents are irrelevant to the method of removing nitrogen oxides (NO _x ) from the off-gas by injecting ammonia gas continuously generated from aqueous urea solution.

According to the aforementioned patent, the hydrolysis reactor pressure is controlled by the heat input into the hydrolysis reactor, the gas take-off rate is controlled by an adjustable control valve and the valve is adjusted to adjust the nitrogen in the combustion gas stream. Adjust the amount required to remove the oxide. Emergency pressure relief means may be on the gas or liquid side of the reactor. In both cases, an outlet may be provided, which is connected to a water-containing dump tank that serves to trap the ammonia gas and prevent the ammonia gas from being released into the atmosphere. Cold water in the dump tank serves to stop the hydrolysis process and prevent the further generation of ammonia.

Summary of the Invention

The present invention

(a) hydrolyzing urea in aqueous solution in a closed reactor to release gaseous ammonia at a rate essentially balanced to the amount required to remove nitrogen oxides from the combustion gas stream; And

(b) contacting the gaseous ammonia with the combustion gas stream.

A method of providing a pressurized gas stream useful for removing nitrogen oxides from a combustion gas stream comprising:

When the ammonia demand required to remove nitrogen oxides suddenly decreases, the reactor is cooled by cooling the solution in the hydrolysis reactor by heat exchange inside or outside the reactor in response to a rapid change in the ammonia demand required to remove the nitrogen oxides. An improved method of providing a pressurized gas stream that maintains the pressure within the preselected range.

The present invention

(a) an aqueous solution of urea or a mixture of urea containing biuret and / or ammonium carbamate with a solids concentration of about 1% to about 76% by weight is fed into the reactor and the temperature or pressure of the reaction mixture is Controlled by heat input to the reactor to hydrolyze the urea at a pressure of about 20 to about 500 psig at a temperature of about 110 ° C. to about 300 ° C. to provide ammonia, carbon dioxide and Producing a residual liquid reaction medium containing a gaseous product stream of water and unreacted urea, biuret and / or ammonium carbamate;

(b) separating the gaseous product stream at a controlled pressure and flow rate;

(c) maintaining a liquid reaction medium in the reactor for further conversion to gaseous ammonia and carbon dioxide and / or recycling at least a portion of the reaction medium to the reactor, urea dissolver or feed solution to the reactor for further conversion. ; And

(d) recovering the gaseous ammonia and carbon dioxide containing product stream and feeding it at a controlled rate approximating the external use demand required to remove the nitrogen oxides for external use.

A method of providing a pressurized gas stream useful for removing nitrogen oxides from a combustion gas stream by selective non-catalytic reduction or selective catalytic reduction comprising:

When the demand for external ammonia suddenly decreases, the pressure in the reactor is reduced by cooling the solution in the hydrolysis reactor by heat exchange in or outside the reactor in response to a rapid change in the ammonia demand required to remove the nitrogen oxides. An improved method of providing a pressurized gas stream is maintained within the aforementioned range.

FIG. 1 is a graph showing the relationship of pressure and temperature over time in a hydrolysis reactor where an immediate interruption of ammonia demand occurs and the urea hydrolysis solution in the reactor is not rapidly cooled by heat exchange.

FIG. 2 is a graph showing the relationship of pressure and temperature over time in a hydrolysis reactor when an immediate interruption of ammonia demand occurs and the urea hydrolysis solution in the reactor is rapidly cooled by heat exchange according to the present invention.

3 is a three-dimensional graph of the pressure in the reactor as a function of cooling rate and liquid level in the reactor.

FIG. 4 is a graph showing the highest pressure in the reactor as a function of the reactor operating temperature without water cooling in the reactor having a volume ratio of gas to liquid of 1 to 2.3.

5 shows a schematic diagram of one embodiment of the present invention.

In the process of the invention, the aqueous urea solution is converted to a gaseous product stream of ammonia and carbon dioxide and used to remove nitrogen oxides from the combustion gas stream by means of an SCR or SNCR NO _x control method. The urea solution is provided at a particular concentration and pumped to the hydrolysis reactor at a controlled rate. The reaction is endothermic and heating is required. The heat input into the reactor is controlled to maintain a constant gas pressure in the reactor. In the reaction, the urea is first hydrolyzed to ammonium carbamate to form a mixture of gaseous ammonia-carbon dioxide product and then fed to the distribution grid in the gas duct at a controlled rate. In some applications, the gas mixture is diluted with air, steam or combustion gas to improve mixing and contact with nitrogen oxides in the combustion gas stream. The gas flow rate is adjusted to match the ammonia rate with the NO _x in the combustion gas stream.

The reaction in the reactor is shown in Scheme 1:

(x) H ₂ O + NH ₂ CONH _2- > 2NH ₃ + CO ₂ + (x-1) H ₂ O

The rate of ammonia generation by the reaction is given by the Arrenhius equation. Thus, the rate of ammonia generation is of the formula Ae- ^{b / kT} , where A is proportional to the number of moles of water and urea, and b is the free energy for the reaction. Controlling the amount of urea solution in the reactor and the temperature of the reactor controls the rate of ammonia generation. Below about 110 ° C., no reaction occurs if no catalyst is used. The present invention contemplates the use of a catalyst that allows temperatures below 110 ° C. In the method of the present invention, the temperature is self-regulated by adjusting the heat input to the reactor to maintain a constant pressure. Increasing the temperature of the reactor by introducing excess heat can accelerate the rate of ammonia generation. Increasing the reactor temperature from 140 ° C. to 158 ° C. increases ammonia generation by 300%.

The rate of NO _x reduction with ammonia produced from urea is the same as pure ammonia. The SAR scheme is shown in Scheme 2:

NO _x removal by ammonia:

4NO + 4NH ₃ + O _2- > 4N ₂ + 6H ₂ O

2NO ₂ + 4NH ₃ + O _2- > 3N ₂ + 6H ₂ O

NO _x removal by urea:

4NO + 2CO (NH ₂ ) ₂ + O _2- > 4N ₂ + 4H ₂ O + 2CO ₂

2NO ₂ + 2CO (NH ₂ ) ₂ + O _2- > 3N ₂ + 4H ₂ O + 2CO ₂

The urea solution may be used in a concentration range of about 1 to 76% and is preferably supplied to the reactor by a volumetric pump at a concentration of 40% or 50%. The urea solution is pumped into the hydrolysis reactor at a controlled rate to maintain a constant level in the reactor. A differential pressure transmitter / regulator that regulates the reactor feed rate is used to monitor the reactor liquid level to maintain a constant liquid level in the reactor. The feed rate is controlled by a proportioning pump or by adjusting the discharge rate from a constant velocity pump equipped with recirculation piping.

A method for generating ammonia at a controlled rate by hydrolysis of urea has been developed for the control of NO _x emissions in SNCR and SCR systems. In this process, heating is applied to cause the hydrolysis reaction. The demand heat input in normal operation is adjusted to meet the production requirements. One means for controlling the heat input is to monitor the pressure in the reaction vessel and adjust the heat to maintain a constant gas pressure as disclosed in the patent application described above. If additional ammonia is required, increase the heat. As the demand for ammonia decreases, the additional amount of heat decreases, and as the solution temperature decreases to satisfy the new demand, the heat stored in the reaction solution and the reaction vessel is depleted. However, if the ammonia demand suddenly or immediately decreases, such as when the amount of oil, coal or gas to be charged to the boiler of a power plant is rapidly reduced, the heat stored in the hot reaction solution and the reaction vessel continues to generate ammonia. As a result, the pressure in the reaction vessel will be excessively increased until the temperature of the solution drops below the ammonia point until the hydrolysis reaction is stopped. In order to limit this increase in pressure, the solution and gas must be withdrawn from the container if the temperature of the solution and the container is not reduced. However, recovery of hot solutions or gases causes major problems that the present invention avoids.

1 and 2 respectively show the pressure in the reaction vessel when no cooling is performed on the aqueous urea solution hydrolyzed in the pressurized reactor (FIG. 1), and when the steam in the heating system of the hydrolysis reactor is replaced with cooling water. The pressure in the reaction vessel of FIG. 2 is shown. According to the present invention, by selecting an appropriate cooling delivery zone for the hydrolysis solution, the vessel is free from the need for recovery of gas or liquid from the reactor, which, upon immediate interruption of the ammonia demand, will result in dangerous release of ammonia to the undesired atmosphere. It is possible to design a system that can control the pressure built up in it and that more economical low pressure reaction vessel designs can be used.

FIG. 1 shows that when an immediate interruption of ammonia demand occurs without applying the cooling means according to the invention to the hydrolysis reactor, the temperature T of the hydrolysis solution drops very slowly, as in the sub-curve of FIG. The pressure P in the interior shows a rapid and continuous increase due to the latent heat in the hydrolysis solution as shown in the upper curve of FIG. 1.

On the other hand, if the hydrolysis solution in the reactor is cooled by heat transfer simultaneously with an immediate interruption of the ammonia demand in accordance with the present invention, the temperature T of the hydrolysis solution drops rapidly as shown in the sub-curve of FIG. ) Quickly drops after reaching a pressure peak. The upper curve in FIG. 2 suggests that the production of gaseous ammonia in the system is essentially stopped.

Figure 3 shows that increasing the gas volume relative to the liquid volume reduces the amount of cooling required to limit the rise in pressure due to the generation of ammonia and carbon dioxide by the heat remaining in the reaction vessel and the heat stored in the solution in the reaction vessel. Shows. This data shows that the pressure increases rapidly when the fill height (liquid level) exceeds 50%.

According to the invention, the use of a heating system in the reactor as a cooling system, the provision of a separate set of cooling coils in the reactor, or a heat exchanger outside the reactor for supplying hot urea solution to the pump and cooling by air or cooling fluid By various means, the hydrolysis solution in the reactor can be cooled. In this way, when the demand for ammonia is reduced, it is possible to rapidly reduce the rate of ammonia generation without the recovery of any high temperature contents (liquid or gas) from the urea reaction vessel in a closed system.

Referring in more detail to the preferred embodiment of FIG. 5, in reactor 10, urea in the feed solution is first hydrolyzed to ammonium carbamate and then to ammonia and carbon dioxide vapor. The concentration of water in the reactor will increase until it is sufficient to provide a balance between discharge and feed solution. Urea hydrolysis reactions are endothermic and require input of heat. The heat input into the reactor for hydrolysis and water evaporation is controlled to be a constant gas pressure. The heat required for the reaction can be supplied in various ways, such as using an external electric resistance heater, an internal electric bayonet heater, or an internal coil using steam or heat transfer fluid.

In general, steam heating is used to provide energy input for the hydrolysis reaction and is the method shown in the process flow diagram of FIG. 5. Typically steam pressures of 20 psig to 500 psig are used.

The reactor 10 is designed to operate at temperatures below 300 ° C. and pressures below 500 psig (the reactor is designed for a maximum pressure of 500 psig). The pressure controller 12 is arranged to control the pressure of the reactor at a set point in accordance with the required transport pressure. The pressure in the reactor 10 is controlled by adjusting the steam flow rate towards the reactor coil 14. The pressure controller 12 is programmed to stop the steam supply and start the water cooling supply when the pressure in the reactor exceeds 220 psig. As a precautionary measure, a pressure switch is provided on the reactor to wire off the steam supply and activate the water cooling system when the pressure exceeds 250 psig.

Ammonia / carbon dioxide vapor steam is discharged from the top of the reactor and sent to the SCR system through piping 16. The ammonia flow rate of the feed duct is controlled by the ammonia demand fixed point signal from the SCR control system 18. Continuously meter the ammonia feed line to maintain the temperature above 75 ° C. It should be noted at this point that CO ₂ and NH ₃ in the product gas can recombine at temperatures below 65 ° C. to form solid ammonium carbonate.

Preferably the heat transfer surface used to heat the reactor for the hydrolysis process is also designed to have a sufficient gas space to liquid space ratio to be suitable to provide the cooling transfer surface required to remove heat from the reactor. In general, the preferred system uses as small a gas volume-to-liquid volume ratio as possible, ie on the order of 0.3 to 10. This ratio must satisfy the requirement that the pressure increase should be limited to below the design pressure of the reactor. The exact ratio for the desired design depends on the cooling medium, the temperature of the medium and the manufacturing economics of the reactor vessel. In fact, it has been found that fill heights on the order of 40 to 60% provide the most economical design. The 50% fill height corresponds to a gas to liquid volume ratio of 1.

The removal of heat from the reactor vessel can be accomplished by a variety of methods, including by circulating the cooling fluid through a heat transfer tube or coil located inside or outside and used to heat the reactor, or by using a separate set of cooling coils. . In addition, cooling can be achieved by, for example, using an external heat exchanger equipped with cooling fins by venting air or other medium through the cooling fins. By appropriate design, the ammonia gas supply can be stopped immediately without the need to evacuate the reactor solution or gas stored in the reactor. This is important to prevent potential leakage of toxic ammonia gas.

In addition to pressure activation, the preferred system also does not remove heat from the reactor vessel and the solution in the reactor whenever there is a sudden decrease in the ammonia demand to close the product exhaust gas pipe, such as when overpressure occurs. Shut off the steam or other heating supply system and start the cooling system. In normal operation, the heat stored in the reactor decreases as ammonia production decreases. This is because the operating temperature of the reactor decreases as the ammonia feed rate decreases. Shutting off the ammonia supply from the reactor at low temperatures results in less pressure and reduced cooling needs, as shown in FIG. 4. The activation process in the preferred system is only cooled if the heat stored in the reactor is high enough so that the blocking of steam or other heating feeds does not prevent pressure buildup and release liquid or gas from the reactor without removing heat from the reactor solution. Start the system.

Another activation method for a cooling system is to continuously compute the real-time average change in pressure versus time and compare the rate of pressure change against the maximum rate of pressure increase. If the rate exceeds the limit, the heating source to the reactor is stopped and the cooling system is started. The system makes it possible to activate the cooling system even if a signal indicating that the ammonia gas outlet line is closed is lost. It is desirable to activate the cooling system at the lowest possible pressure that does not affect the normal operation of the conversion system of urea to ammonia.

As an example, if a reactor designed to produce ammonia from urea at a rate of 500 lb / hr is producing ammonia at an operating temperature of 152 ° C., the ammonia emissions are blocked without first reducing ammonia production, Activated to remove heat from the reactor and stop ammonia production. The resulting ammonia will recombine with carbon dioxide in the reactor with adequate cooling, and the pressure in the reactor will be below atmospheric pressure. Running the same reactor at a reduced production rate at an operating temperature of 135 ° C. will not activate the cooling system. At this time, only the heat will be blocked and the pressure will stabilize.

If the water cooling system fails to prevent overpressure, a pressure relief valve set at about 300 psig is provided in the reactor to drain the solution into the supply tank or continuous dissolver. The pressure relief valve drains the solution supply tank to a low liquid level and stops the ammonia generation while rapidly cooling the pressure relief liquid solution and / or gas. The solution feed tank or continuous dissolver is equipped with a low level switch to stop the reactor if the solution in the solution feed tank is less than the solution in the reactor. This prevents the reactor feed pump from running out of solution, prevents the reactor from drying, and ensures that there is always enough solution to cool the reactor solution in case of emergency overpressure.

Claims (20)

(a) hydrolyzing urea in aqueous solution in a closed reactor to release gaseous ammonia at a rate consistent with the amount required to remove nitrogen oxides from the combustion gas stream; And (b) contacting the gaseous ammonia with the combustion gas stream. A method of providing a pressurized ammonia-containing gas stream for removing nitrogen oxides from a combustion gas stream comprising: When the ammonia demand required to remove nitrogen oxides suddenly decreases, by cooling the solution in the hydrolysis reactor by heat exchange inside or outside the reactor in response to a rapid change in the ammonia demand required to remove the nitrogen oxides. Maintaining the pressure in the reactor within a preselected range. The method of claim 1, Wherein the hydrolysis reactor comprises a heat exchanger suitable for both heating and cooling of the solution, wherein cooling is performed by passing the coolant through the heat exchanger. The method of claim 1, Wherein the hydrolysis reactor comprises separate heating and cooling means, and heat exchange to cool the solution is carried out by passing the coolant through the means. The method of claim 1, The hydrolysis reactor is equipped with an external heat exchanger, and heat exchange to cool the solution is carried out by withdrawing the solution from the reactor, passing the solution through the external heat exchanger, and sending the cooled solution to the reactor for further use. Way. The method of claim 1, Wherein the hydrolysis reactor is designed in the range of 0.3 to 10 gas / liquid volume ratio. The method of claim 1, Wherein the gas / liquid volume ratio of the hydrolysis reactor is selected such that the heating surface for the hydrolysis reaction provides a sufficient cooling surface for heat transfer to remove storage heat from the reactor. The method of claim 1, A hydrolysis reactor equipped with a cooling system is activated by a pressure switch when the reactor pressure reaches a fixed value. The method of claim 1, Wherein the hydrolysis reactor is equipped with a cooling system that is activated when the temperature of the reactor upon closure of the ammonia gas exhaust line exceeds a predetermined fixed temperature. The method of claim 1, Wherein the hydrolysis reactor is equipped with a cooling system that is activated by both the operating pressure and the temperature of the reactor upon closure of the ammonia gas exhaust piping. The method of claim 1, Wherein the hydrolysis reactor is equipped with a cooling system that is activated by an increasing rate of pressure in the reactor above a predetermined value. (a) an aqueous solution of unreacted material selected from urea with a solids concentration of 1% to 76% by weight, urea mixed with biuret, urea mixed with ammonium carbamate, urea mixed with ammonium carbamate and unreacted material And the temperature or pressure of the reaction mixture is usually controlled by heat input into the reactor to hydrolyze the urea at a temperature of 110 ° C. to 300 ° C. and a pressure of 20 to 500 psig for external use in step (d). Producing a gaseous product stream of ammonia, carbon dioxide and water and a residual liquid reaction medium containing one of said materials at a rate sufficient to (b) separating the gaseous product stream at a controlled pressure and flow rate; (c) maintaining a liquid reaction medium in the reactor for further conversion to gaseous ammonia and carbon dioxide and recycling all or part of the reaction medium to the reactor, urea dissolver or feed solution to the reactor for further conversion; And (d) recovering the gaseous ammonia and carbon dioxide containing product stream and feeding it at a controlled rate of approximate amount necessary for the external use demand required to remove nitrogen oxides for external use. A method of providing a pressurized gas stream useful for removing nitrogen oxides from a combustion gas stream by selective non-catalytic reduction (SNCR) or selective catalytic reduction (SCR) comprising: When the demand for external ammonia suddenly decreases, the pressure in the reactor is reduced by cooling the solution in the hydrolysis reactor by heat exchange in or outside the reactor in response to a rapid change in the ammonia demand required to remove nitrogen oxides. How to keep mine. The method of claim 11, Wherein the hydrolysis reactor is designed in the range of 0.3 to 10 gas / liquid volume ratio. The method of claim 11, Wherein the gas / liquid volume ratio of the hydrolysis reactor is selected such that the heating surface for the hydrolysis reaction provides a sufficient cooling surface for heat transfer to remove storage heat from the reactor. The method of claim 11, A hydrolysis reactor equipped with a cooling system is activated by a pressure switch when the reactor pressure reaches a fixed value. The method of claim 11, Wherein the hydrolysis reactor is equipped with a cooling system that is activated when the temperature of the reactor upon closure of the ammonia gas exhaust line exceeds a predetermined fixed temperature. The method of claim 11, Wherein the hydrolysis reactor is equipped with a cooling system that is activated by both the operating pressure and the temperature of the reactor upon closure of the ammonia gas exhaust piping. The method of claim 11, Wherein the hydrolysis reactor is equipped with a cooling system that is activated by an increasing rate of pressure in the reactor above a predetermined value. The method of claim 11, Wherein the hydrolysis reactor comprises a heat exchanger suitable for both heating and cooling of the solution, wherein cooling is performed by passing the coolant through the heat exchanger. The method of claim 11, Wherein the hydrolysis reactor comprises separate heating and cooling means, and heat exchange to cool the solution is carried out by passing the coolant through the means. The method of claim 11, The hydrolysis reactor is equipped with an external heat exchanger, and heat exchange to cool the solution is carried out by withdrawing the solution from the reactor, passing the solution through the external heat exchanger, and sending the cooled solution to the reactor for further use. Way.